Field of the Invention
The invention relates to a method for detecting combustion misfires, and in particular to a method that makes it possible to detect misfires with high reliability even when combustion misfires occur in a plurality of cylinders.
In order to detect combustion misfires, methods that evaluate the fluctuations in crankshaft rpm caused by the misfire are employed, among others. One such method is described in German Published, Non-Prosecuted Application DE 41 00 527 A1, for example. In that method, so-called engine roughness values are calculated, in which disturbances that could lead to misdetections, such as acceleration or deceleration of the vehicle, are already more or less well compensated for. Those engine roughness values are then compared with threshold values, and if those threshold values are exceeded, a combustion misfire is detected. Even though the engine roughness values are calculated in a sometimes quite complicated way, which is intended to preclude the disturbances as much as possible, misdetections repeatedly occur, or in other words, combustion misfires are indicated even though in fact none are actually occurring, or else no combustion misfires are indicated even though in fact they are occurring. The danger of non-detection is especially high if combustion misfires are occurring in a plurality of cylinders, or more specifically, if in a four-stroke internal combustion engine, combustion misfires occur in a plurality of cylinders during two rotations of the crankshaft. The term "multi-cylinder misfires" is used below to describe such an occurrence. In multi-cylinder misfires, the engine roughness values are less than with single-cylinder misfires. The threshold values must therefore be lowered in order to still detect such multi-cylinder misfires. One such method is described in German Published, Non-Prosecuted Application DE 41 18 580 A1. In the method described therein:
engine roughness values for single-cylinder misfires are determined in an arbitrary way; PA1 a current engine roughness threshold value at a given time, which is valid for single-cylinder misfires, is determined as a function of the current operating state of the engine; PA1 the engine roughness threshold value is lowered if multi-cylinder misfires are detected; and PA1 a conclusion is drawn that misfires are occurring in the applicable cylinder if the engine roughness value for that cylinder exceeds the lowered engine roughness threshold value. Multi-cylinder misfires are then detected as follows: PA1 engine roughness values for single cylinders are determined in an arbitrary way; PA1 a predetermined number of summation terms that each come with the same algebraic sign are engine roughness values or differences between such values, which are added together; and PA1 a conclusion is drawn that a multi-cylinder misfire is occurring if at least one sum exceeds a predetermined threshold sum value.